| The distributed generation which use wind power,solar energy and other intermittent renewable energy is an important way to solve energy problems.There is no need for power grid reconstruction when the permeability of distributed renewable energy sources is low,while more and more load is consumed in local,the power system ability to cope with load growth has been improved,and it delay the upgrading of distribution power grid.With the large-scale application of distributed generation technology,the permeability of distributed renewable energy in the power system is higher and higher,since the distributed power generation unit with the interface of the power electronic converter does not have the inertia and damping of the synchronous generator,the rotation reserve capacity and the rotational inertia in the system are relatively reduced,at this point the power system is vulnerable to power fluctuations and system failures.The distributed virtual synchronous generator has the technical advantages of simulating the rotational inertia,damping characteristic and droop characteristic of synchronous generators.So that the distributed virtual synchronous generator can not only provide energy for the grid,but also reduce the negative impact of t the large scale application of distributed generation unit.The distributed virtual synchronous generator can provide some support to power grid,and provide a new solutions for the large-scale application of distributed generation.The distributed virtual synchronous generator generally connect to the low and medium voltage distribution network,access locations are the end of the distribution network,where the power grid environment is relatively poor,the short circuit failure and grid voltage imbalance often occur.The operating mechanism and output performance of distributed virtual synchronous generator not only related to the utilization of renewable energy,but also to the safe and stable operation of the power grid.When the distributed virtual synchronous generator working in the islanding model,there are many technical problems in unbalanced load control technology and parallel control technology.Therefore,the paper studies the power grid adaptability,load adaptability and parallel control technology of distributed virtual synchronous generator,are not only related to its own safe and reliable operation,but also for the construction of high reliability,high permeability power grid,and it has a very important theoretical research value and practical significance.With the support of International Science & Technology Cooperation Program of China “The Power Quality Control Mechanism and Key Technology of Distributed Photovoltaic Micro-network” and High School Science and Technology Floor Project of Jiangxi Province “The key technology research of PV micro-network intelligent controller”,the paper carries out research in the distributed virtual synchronous generator grid adaptability,load adaptability and parallel control technology,which including the low voltage ride-through technology of distributed virtual synchronous generator,the control technology of distributed virtual synchronous generator under unbalanced grid voltage,the control technology of distributed virtual synchronous generator with unbalance load,the control technology of paralleled distributed virtual synchronous generator.The main work and innovation is reflected in the following points.(1)The paper summarized the background,definition and characteristics of distributed generation,summed up the existing control strategy of distributed inverter power,and conclude and summarized the development process and the research status of the virtual synchronization control strategy.(2)The paper summarized the basic principles of virtual synchronous generators,introduced the main structure and control strategy of the virtual synchronous generator in detail,and proposed a voltage and current control loop design method based on synchronous rotation dq coordinates of voltage-based virtual synchronous generator voltage,which lay the foundation for the improvement of distributed virtual synchronous generator control strategy.A small signal model is established for the virtual synchronous generator,the influence of the main control parameters on the stability and dynamic performance of the virtual synchronous generator is analyzed by the root locus method,and the specific method of the control parameter setting is given.(3)The low voltage ride-through control technology of distributed virtual synchronous generator is proposed based on the analysis of the operating characteristics of the traditional virtual synchronous generator in the short circuit fault of the power grid.In the improved virtual synchronization control strategy,the introduction of virtual impedance limits the instantaneous current in synchronous rotation coordinates.The suppression of negative sequence current is achieved with positive and negative sequence control of current respectively,when the asymmetric short circuit fault occurred.The positive voltage command value are get with the traditional virtual synchronous generator power control loop,while the negative sequence current command value is set to 0.The PI controller is introduced to improve the reactive power control loop of the virtual synchronous generator,it improve the reactive power response speed to meet the reactive current injection rate requirements of low voltage ride-through.Finally,the calculation method of the active power and reactive power command value is given.Improved distributed virtual synchronous generator low voltage ride-through control technology do not change the virtual synchronous generator mechanism,there is no need to change the control strategy when the power grid short circuit failure occur,and the implementation is simple.(4)When the power grid voltages are unbalanced,the output current imbalance and the output power fluctuation of the distributed virtual synchronous generator,in accordance with these problems,the positive and negative sequence control strategies of virtual synchronous generator are proposed,the three different control objectives achieve respectively by introducing the current compensation amount according to the instantaneous power model,the three control objectives are achieving the output current balance,inhibiting fluctuations of active power,inhibiting the fluctuations of reactive power.The optimization coefficient is used to adjust the value of positive and negative sequence current compensation,the transition and mutual transformation of control objectives are achieved,and realize the output performance multi-objective optimization control of the virtual synchronous generator when the grid voltage is unbalanced,the improved control strategy improve the adaptability of distributed virtual synchronous generators under unbalanced power grids.(5)The performance of the virtual synchronous generator with unbalanced load,the power distribution characteristics and the circulation current characteristics of the parallel operation are analyzed.On this basis,a virtual synchronous generator voltage sequence control strategy based on virtual complex impedance technology is proposed.In the improved control strategy of virtual synchronous generator with unbalanced load,the negative sequence of terminal voltage is suppressed by introducing the negative sequence control loop of terminal voltage,the load voltage balance is realized using the virtual impedance,the virtual impedance compensate voltage drop of transmission line.Aiming at the power distribution and circulation current problem of parallel virtual synchronous generator,the negative feedback of load voltage is introduced in reactive power loop to realize the decoupling of reactive power allocation and transmission impedance.The transmission impedance is set to pure inductive load with the virtual complex impedance,in this way,the power coupling and the impact of the line impedance difference on power distribution and circulation current are reduced.The influences of control parameters and virtual complex impedance values on the equivalent output impedance and transmission impedance of the virtual synchronous generator are analyzed theoretically,and the control loop parameters and virtual complex impedance design principles are obtained.The power distribution and circulation current suppression of the parallel virtual synchronous generator with unbalanced load are achieved with the control strategy combined with the above two kinds of control methods.(6)The digital simulation platform and physical experiment platform are constructed respectively in the Matlab/Simulink simulation software and the laboratory,the relevant theoretical analysis and the proposed control strategy are verified.Simulation and experimental results show that the proposed control strategy has a good support for improving the power grid adaptability,load adaptability and parallel output performance of distributed virtual synchronous generator. |
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